Field of the Invention
The present invention is related to antennas, and, more particularly, to helical antennas for use in GPS receivers.
Description of the Related Art
Multipath error is currently one of the most important contributions to the GNSS positioning error budget when a signal reflected from the underlying ground surface is received at the output of the receiving antenna along with the line-of-sight signal. Multipath error is proportional to the ratio
      DU    ⁡          (      θ      )        =                    F        ⁡                  (                      -            θ                    )                            F        ⁡                  (          θ          )                      .  
This ratio is typically called Down/Up ratio. Here, θis the elevation angle over the local horizon, and F(+/−θ) is the directional diagram (DD) for the antenna at angle θover and under the local horizon respectively. To reduce multipath error, the value F(−θ) should be small. However, to provide stable and reliable operation of a positioning system, reception of all signals over the local horizon is needed.
Hence, to enhance accuracy of positioning systems, one needs to develop and design receiving antennas with Π-shaped (rectangular) DD providing antenna gain close to a constant value in the whole upper hemisphere and forming a sharp drop when crossing the local horizon downward.
Navigation signals are received from satellites in the upper hemi-sphere up to elevations 10° . . . 15° from the horizon. A signal reflected from the ground is fed from the lower hemi-sphere side. FIG. 1 shows a conditional division of space into upper (front) and lower (back) hemi-spheres, as well as a schematic diagram of the direct and reflected waves. To provide both navigation signal reception in the whole upper hemi-sphere and suppression of signals reflected from the ground, the antenna needs a high DD level in the upper hemi-sphere, a low DD level in the lower hemi-sphere, and a sharp drop of DD to the horizon direction.
A quadrifilar helix antenna is known (see Josypenko, CAPACITIVELY LOADED QUADRIFILAR HELIX ANTENNA, U.S. Pat. No. 6,407,720), with capacitive elements incorporated in spiral turns as shown in FIG. 2.
This antenna is produced as a dielectric cylinder 206 with mylar tapes 202, 203, 204, 205 being wound on it. The tapes are both-side-metallized, such that metallization areas 301-302−321-320 on different sides of the tape would be overlapped, forming capacitors C1-C19.
U.S. Pat. No. 6,407,720 discloses that the area of capacitor plates is maximum at excitation point 201 and then reduces according to the exponential law to the minimum value at the end of the spiral. One of the embodiments shows that the winding angle is constant and equal to 66.64° (see column 8, line 15 in U.S. Pat. No. 6,407,720).
In the proposed antenna this angle can be varied.
Known prior art solutions do not allow obtaining a sharp drop in DD in the direction of the horizon.
FIG. 4 shows an exemplary DD taken from U.S. Pat. No. 6,407,720. Unlike FIG. 1 the horizon direction is zero of elevation angles. The corresponding angles reading from the horizon (see FIG. 1) are in italics. In this figure, 401 is the directional diagram of a spiral antenna with turns in the form of simple metal tapes; 402 is the directional diagram of the spiral antenna with capacitor spiral turns (the subject matter of U.S. Pat. No. 6,407,720).
In FIG. 4: θ=0° is the direction to the local horizon; θ=10°, θ=−10° are the directions that differed by 10° from the horizon direction up and down respectively. DD values in the mentioned directions are: F(10°)=0.95, F(−10°)=0.85. Hence for the given antenna at the elevation of 10°, the Down/Up ratio is as follows: DU(10°)[dB]=20log[F(−10°)/F(10°)]=−0.97 dB, which is clearly inadequate for GPS applications, where at least −20 dB is required to suppress signals reflected from the ground.